The present invention relates to an offshore rig suitable for conducting mineral exploration and production operations, and more particularly to a self-elevating jack-up rig incorporating a releasing mechanism for a rack chock.
Jack-up rigs have been extensively used as the types of offshore working platforms especially suitable for deployment in medium water depth. Generally, this type of rig uses jackable legs for supporting a working platform, or hull at an elevated position in a selected location. When a jack-up rig is towed to a desired location, the legs extend upwardly through wells in the hull so as not to interfere with the towing. Once the rig is delivered to the selected location, the legs are lowered to the bottom of an ocean, and powerful jacks elevate the hull to an operating level above the waves.
The hull and the legs are rigidly engaged with each other through a system of rack teeth on the supporting legs and opposed, matching rack sections of rack chocks. The rack chocks are moved horizontally and vertically to allow a mating alignment with the leg chords. When the alignment is completed, the rack chock rigidly connects the hull to the legs of the rig. During operation of the rig, the legs and the hull remain locked in their unitary connection, resisting overturning moments imposed on the legs by wave forces.
When the legs and the hull are rigidly engaged, considerable pressures act on the surfaces of the meshed teeth of the leg chords and the rack chocks. In effect, the teeth become locked, and withdrawal of the rack chock from engagement with a leg chord becomes extremely difficult.
Still, under certain circumstances, such when repositioning the hull or the entire rig, the engagement between the legs and the hull must be broken. Oftentimes, conventional methods and equipment cannot generate sufficient force to release a "frozen" jack.
Heretofore, various methods have been employed for releasing of a screw jack. One of the conventional methods involves burning of the jack bearing plate or cap in order to break the rigid engagement between a leg chord and a rack chock. This method is labor-intensive, time-consuming and relatively expensive. Additionally, the cap must be replaced before the screw jack is moved laterally into an engagement with the leg teeth to reinstate the rigid engagement of the hull to the legs.
Other known techniques involve the use of a wedge-shaped device, wherein an inclined surface of the wedge facilitates release of the chock mechanism. This design is disclosed in U.S. Pat. Nos. 5,486,069 and 5,611,645 issued to John O. Breeden. Both patents disclose a fixation system that uses a toothed chord chock, which moves both horizontally and vertically. The toothed rack chock has upper and lower inclined surfaces. The fixation system includes an upper and lower wedge, which rides on upper and lower fixed inclined surfaces. The upper and lower wedges move to engage the upper and lower inclined surfaces of the rack chock and release the engagement between the leg chord and the rack chock. While such design can work satisfactory in many cases, there exists a need for a more straightforward design for releasing of a rack chock.
The present invention contemplates elimination of drawbacks associated with prior designs and provision of an improved mechanism for releasing of a rack chock from its rigid engagement with a leg chord.